The use of plasma-enhanced processes in the manufacture of semiconductor-based products (such as integrated circuits or flat panel displays) is well known. Generally speaking, plasma-enhanced processes involve the processing of a substrate (e.g., a glass panel or a semiconductor wafer) in a plasma processing chamber. Within the plasma processing chamber, a plasma may be formed out of appropriate etchant or deposition source gases to respectively etch or deposit a layer of material on the surface of the substrate.
FIG. 1 depicts a capacitively-coupled plasma processing chamber 100, representing an exemplary plasma processing chamber of the types typically employed to etch a substrate. A chuck 104 represents the workpiece holder on which a substrate 106 is positioned during etching. The chuck 104 may be implemented by any suitable chucking technique, e.g., electrostatic, mechanical, clamping, vacuum, or the like. During etching, the chuck 104 is typically supplied with RF power having a frequency of, for example, about 400 Khz to about 27 Mhz, by an RF power supply 110. In some systems, chuck 104 may be supplied with dual frequencies, e.g., 2 MHz and 27 MHz simultaneously during etching.
A reactor top 112, formed of a conductive material such as aluminum, is disposed above substrate 106. Confinement rings 102 may be coupled in a fixed manner to reactor top 112 or may be coupled to cam-based plungers (not shown in FIG. 1) that allow confinement rings 102 to be raised and lowered without moving reactor top 112.
In general, confinement rings 102 help confine the etching plasma to the region above substrate 106 to improve process control and to ensure repeatability. Although only two confinement rings are shown in the example of FIG. 1, it should be understood that any number of confinement rings may be provided.
An upper electrode 114 and a baffle 116 are also coupled to reactor top 112. The upper electrode 114 may be grounded (as in the case of FIG. 1) or may be powered by another RF power source 120 during etching. If the upper electrode 114 is powered, it may be insulated from the remainder of the reactor to isolate the electrode from ground. During etching, plasma is formed from etchant[so]urce gas supplied via a gas line 122 and the baffle 116.
When RF power is supplied to the chuck 104 (from the radio frequency generator 110), equipotential field lines are set up over the substrate 106. During plasma processing, the positive ions accelerate across the equipotential field lines to impinge on the surface of substrate 106, thereby providing the desired etch effect (such as improving etch directionality). Due to geometry factors, however, the field lines may not be uniform across the substrate surface and may vary significantly at the edge of substrate 106. Accordingly, a focus ring is typically provided to improve process uniformity across the entire substrate surface. With reference to FIG. 1, chuck 104 is shown disposed within a focus ring 108, which is typically formed of a suitable dielectric material such as ceramic, quartz, or plastic.
The equipotential field lines that are set up during plasma etching may be seen more clearly in FIG. 1B. In FIG. 1B, the presence of focus ring 108 allows the equipotential field lines to be disposed substantially uniformly over the entire surface of the substrate, thereby allowing etching to proceed in a uniform manner across the substrate. As seen by FIG. 2, however, some of the equipotential field lines also extend into the region 160 outside of focus ring 108. The presence of the equipotential field lines in region 160 may cause any charged particles that leak past the confinement rings to accelerate in a direction perpendicular to the equipotential field lines toward the chamber walls. This acceleration and the subsequent collision between the charged particles and the chamber walls may generate secondary electrons, which may ignite and/or sustain unconfined plasma in the region 160 (i.e., unintended plasma that is not confined to region directly above the substrate).
Furthermore, current return paths have relied on the chamber wall 118 for a return path or a return path outside the chamber. Magnetic fields are generated from the return paths and cause magnetic fields that can light and sustain a plasma outside the confined region. The dotted lines in FIGS. 1A and 1B illustrate the current return path along the chamber wall 118.
The inadvertent generation of plasma in the region 160 renders the etch process difficult to control and may damage components within this region. By way of example, this unconfined plasma, which may be unplanned and/or intermittent, changes the location of power absorbed by the plasma within the plasma processing chamber, thereby making it difficult to control the delivery of power to the chuck to achieve consistent, repeatable etch results. As another example, the presence of unwanted plasma in region 160 may cause damage to the chamber door (not shown), particularly to the seals that are provided therewith. The chamber door is necessary for substrate transport into and out of the chamber, and if the seals are damaged, accurate control of the chamber pressure may be difficult. When the seals and/or other components in the region 160 are inadvertently attacked by the plasma, particulate and/or polymeric contaminants may form along the chamber walls, potentially leading to contamination of the etch environment.
Accordingly, it would be desirable to provide techniques for minimizing and/or eliminating the unwanted plasma formation in the region outside of the focus ring of the plasma processing chamber.